Cardiac pacing leads are well known and widely employed for carrying pulse stimulation signals to the heart from a battery operated pacemaker, or other pulse generating means, as well as for monitoring electrical activity of the heart from a location outside of the body. Electrodes are also used to stimulate the heart in an effort to mitigate bradycardia or terminate tachycardia or other arrhythmias. In all of these applications, it is highly desirable to optimize electrical performance characteristics of the electrode/tissue interface. Such characteristics include minimizing the threshold voltage necessary to depolarize adjacent cells, maximizing the electrical pacing impedance to prolong battery life, and minimizing sensing impedance to detect intrinsic signals.
Pacing (or stimulation) threshold is a measurement of the electrical energy required for a pulse to initiate a cardiac depolarization. The pacing threshold may rise after the development of a fibrous capsule around the electrode tip, which occurs over a period of time after implantation. The thickness of the fibrous capsule is generally dependent upon the mechanical characteristics of the distal end of the lead (i.e., stiff or flexible), the geometry of the electrode tip, the microstructure of the electrode tip, and the biocompatibility of the electrode and other device materials. In addition, the constant beating of the heart can cause the electrode to pound and rub against the surrounding tissue, causing irritation and a subsequent inflammatory response, eventually resulting in a larger fibrotic tissue capsule.
In a pacemaker electrode, minimal tissue reaction is desired around the tip, but firm intimate attachment of the electrode to the tissue is essential to minimize any electrode movement. A porous electrode tip with a tissue entrapping structure allows rapid fibrous tissue growth into a hollow area or cavity in the electrode tip to facilitate and enhance attachment of the electrode to the heart and increase biocompatibility. A reduced electrode dislodgement rate is also expected as a result of such tissue in-growth. A further aspect is selection of the average pore size, which must accommodate economical construction techniques, overall dimensional tolerances, and tissue response constraints. Tissue in-growth may assist in anchoring the electrode in place and increasing the contact surface area between the electrode and the tissue.